1. Field of the Invention
The present invention relates to production methods for sintered bearing members, fluid dynamic pressure bearing devices, and spindle motors, and in particular relates to production techniques for sintered bearings which are used in spindle motors rotating with high precision for information devices.
Bearing devices using sintered bearing members have been used for spindle motors installed in optical disk devices in the field of information devices and for spindle motors used for polygonal mirrors installed in laser printers, and the like. Recently, these bearing devices have been also used for spindle motors installed in magnetic disk devices (hard disk drives). Fluid dynamic pressure bearing devices are used for spindle motors installed in magnetic disk devices as the bearing devices, and are required to have higher levels of performance such as high precision rotation, low noise, shock resistant, and low bearing torque loss even in the case of using sintered bearings. Therefore, several techniques have been researched for sintered bearing members to efficiently generate dynamic pressure.
The following techniques regarding the fluid dynamic pressure bearing devices have been proposed. For example, Japanese Patent Application Laid Open No. 7-216411, in the Abstract, proposes a technique to reduce friction coefficient and improve sliding performance, in which pores of at least a sliding surface of a sliding member is sealed by an impregnant to reduce the frictional resistance to sliding, thereby improving the frictional performance of the sliding member. Japanese Patent Application Laid Open No. 11-62948 proposes a technique to reduce production cost and ensure high precision rotation, in which a bearing surface facing an outer surface of a shaft is formed on an inner surface of a bearing body made from a sintered alloy, and is formed with dynamic pressure grooves inclined toward the axial direction of the bearing body, whereby the shaft is supported in a floating condition by air dynamic pressure generated by relative rotation of the shaft and the bearing body.
Japanese Patent Application Laid Open No. 2002-333023 proposes a technique to produce sintered sliding bearings with few processes, high quality, and stable yield performance, in which a resin is impregnated into pores of a sintered body and hardened therein; the sintered body is inserted into a die and compressed to reduce or close by plastic working the clearance formed between the pore and the resin due to shrinking during hardening of the resin.
Conventional fluid dynamic pressure bearings obtained by the techniques in the above proposals can ensure precise high speed rotation and low noise compared to ball bearings. Furthermore, the bearings can support large loads by suitable design and forming of the dynamic pressure grooves on the bearing surface. The dynamic pressure grooves on the bearing surface are formed in a sintered bearing member by plastic working. The sintered bearing member is produced by compacting a metal powder and sintering a green compact, and is a porous material usually having about 20% porosity, whereby lubricating oil is impregnated into pores, and the bearing member is used in several kinds of motors. The fluid dynamic pressure bearing device used for spindle motors which is required to have precision rotation as mentioned above is formed with dynamic pressure grooves to generate dynamic pressure on a bearing surface, and a member rotating relative thereto is supported by the dynamic pressure.
An ordinary sintered bearing member made of a porous material has a disadvantage that the dynamic pressure generated on a bearing surface leaks through pores, thereby decreasing load supporting capacity. As means for preventing leakage of the dynamic pressure, it has been known to impregnate a resin into pores and seal in a production process of the sintered bearing member. By such a treatment, decrease of the dynamic pressure generated on the bearing surface is avoided and the rotating member can rotate with high precision.
When the dynamic pressure grooves are formed on the bearing surface, for example, in a case of a sintered bearing member having multiple circular arc grooves, such as three circular arc grooves shown in FIG. 3, a pin having protrusions corresponding to the three circular arc grooves is inserted into an inner peripheral surface of a ring-shaped sintered material into which a resin was impregnated, and the pin is extracted therefrom, whereby the shape of the three circular arc grooves is transferred to the inner peripheral surface of the sintered material. For example, in a case of a sintered bearing member having herringbone grooves shown in FIG. 4, a pin having protrusions corresponding to the herringbone grooves is inserted into an inner peripheral surface of a ring-shaped sintered material into which a resin was impregnated, and the outer peripheral surface of the sintered material is compressed to abut the inner peripheral surface to the pin, whereby the shape of the herringbone grooves is transferred to the inner peripheral surface of the sintered material.
When the three circular arc grooves or the herringbone grooves are repeatedly formed by the above-mentioned plastic working in the ring-shaped sintered material into which a resin was impregnated, the resin impregnated into the sintered material may adhere to the surface of the pin which forms the dynamic pressure grooves. If the resin adheres to the surface of the pin which forms the dynamic pressure grooves, the shape of the dynamic pressure grooves is not precisely transferred to the inner peripheral surface of the sintered material. Furthermore, scratches may be formed on the inner peripheral surface of the sintered material when the pin is extracted during the forming of the three circular arc grooves in the sintered material, and the scratches may result in leakage of dynamic pressure.